For example, in Japanese Patent Application Laid-Open Publication No. 2003-298181, an optical transmission circuit which suppresses variations in the extinction ratio of output light of a laser caused by variations in laser characteristics is described. Specifically, a composition, etc. for detecting optical signals from a laser diode by a photodiode and changing the bias current of the laser diode in accordance with a DC component of an output of the photodiode is disclosed. By virtue of this, the temperature variations occurring in the threshold current of the laser diode can be compensated for.
In recent years, along with improvements in the communication speed, the communication speed undergoes transition from 10 Gbps to 25 Gbps, 40 Gbps, etc. Along with such improvements in the communication speed, application of optical communication devices supporting optical fiber cables as, for example, router devices or server devices is advancing. The optical communication devices usually presuppose the transmission in a long distance on the order of, for, kilometers between devices, and it is important to ensure reliability with this transmission distance. For example, in a laser diode serving as a transmission element of optical signals, variations in the light output intensity caused in relation to temperature (in other words, reduction in reliability) are problems. Therefore, for example, like the one Japanese Patent Application Laid-Open Publication No. 2003-298181 (Patent Document 1), a method in which, with respect to a laser diode, a monitoring photodiode which detects the light output intensity the laser diode is disposed in the vicinity of the laser diode so as to suppress the variations in the light output intensity is used.
FIG. 15 is a circuit block diagram illustrating an example of a composition of an optical communication module studied as a premise of the present invention. The optical communication module illustrated in FIG. 15 includes a laser diode block LDBK including a laser diode LD and a monitoring photodiode PDm, a laser diode driver circuit LDD1 which drives LD, a current/voltage converter circuit IVC which converts a current from PDm to a voltage and averages it, and an error amplifier circuit EA. LDBK is realized, for example, by one package.
LDD1 includes a variable current source CSV which causes a bias current to flow and a driver circuit DV which controls the current of LD by adding or subtracting an AC current to or from the bias current of CSV in accordance with the logic of an input signal Vin. LD emits light in accordance with the magnitude of the current, and the intensity of the light emission is detected as a current flowing to PDm. IVC averages the current of PDm as a voltage, and EA controls the current value of CSV so that the averaged voltage (i.e., it indicates average light emission intensity) becomes constant. When the method like that of FIG. 15 is used, the above-described variations in the light output intensity caused in accordance with temperature can be suppressed. To further enhance reliability, for example, it is conceivable to add a Peltier element for maintaining a constant temperature and a thermistor or the like for controlling the Peltier element to the inside of the package of the laser diode block LDBK.
On the other hand, the above-described optical communication devices include many kinds of devices having comparatively large sizes (for example, on the orders of several tens of centimeters and meters), and communications using electric signals are usually carried out in the devices. Therefore, in the optical communication devices, for example, optical signals input from outside are converted to electric signals, predetermined processes are carried out while carrying out short-distance communications (for example, on the order of meters) by the electric signals in the devices, and the electric signals are converted to optical signals again and output to the outside. The short-distance communication is carried out by using, for example, copper cables; however, as the communication speed is increased, the quality of transmission waveforms is extremely lowered in the case of using the copper cables. Therefore, application of optical communications to the short-distance communications in such devices is required.
Therefore, it is conceivable to install an optical communication module including a laser diode block LDBK such as that described above at the part of each output interface in such a device. However, since the number of parts is large in the laser diode block LDBK as described above, the cost is increased, and ensuring a mounting area of the parts is not easy. Thus, when viewed from the user side, upon change from the copper cables to optical fiber cables, realizing an optical communication device by replacing only the part of input/output interfaces with respect to an existing device instead of newly introducing a whole device is desirable in terms of cost, etc.
In this case, the laser diode block LDBK, etc. have to be mounted within the range of the limited mounting area where the part of input/output interfaces is present in the existing device; therefore, the number of parts is desired to be reduced as much as possible. Even in the case in which LDBK is composed of LD and PDm, a mechanism or the like which outputs the light emitted from LD to the two directions for communication and for monitoring is required; therefore, the size of LDBK becomes comparatively large.